Regenerative brake systems in motor vehicles make it possible for at least a portion of the energy applied during braking to be stored in the vehicle and re-used for driving the vehicle. As a result, the energy consumption of the vehicle overall can be lowered and the operation can therefore be made more economical. Motor vehicles with a regenerative brake system generally have different types of brake actuators for this purpose.
In this context, hydraulic friction brakes such as are known from conventional motor vehicles and an electrically regenerative brake are generally used. The brake pressure for the friction brakes is applied via a brake pressure generating means or via the brake pedal movement as in conventional friction brakes. The electrically regenerative brake is generally embodied as an electric machine/machines actuated in the generator mode and via which at least a portion of the entire braking power is applied. The acquired electrical energy is fed into or back to a storage medium such as, for example, an on-board battery and re-used for driving the motor vehicle, wherein either suitable actuation of the electric machine/machines or a number of at least one separate electric motors is used.
Conventional brake systems which exclusively have friction brakes decelerate the vehicle depending on the position of the brake pedal and the pressure of a braking means is built up with or without auxiliary energy, which braking means is taken up by the friction brake or brakes which subsequently apply a corresponding braking torque to the wheels. The pedal position therefore corresponds to the braking behavior of the motor vehicle. Exceptions may occur in the case of interventions by electronic safety systems such as the electrical stability program (ESP), which can comprise devices for automatically building up pressure irrespective of the brake pedal position.
This clear relationship between the pedal position and the braking behavior is not provided without further measures in the case of the drag torque which is applied by a generator. WO 2004/101308 A1 discloses a brake system and a method for closed-loop controlling a brake system of a motor vehicle which has an electrically regenerative brake, in particular a generator and a number of hydraulic friction brakes driven by at least one brake pressure generating means via a braking medium, the overall deceleration of which is made up of deceleration portions of the friction brakes and of the electrically regenerative brake. In order to achieve the highest possible level of braking comfort and a pleasant braking sensation, braking medium is diverted into a low-pressure accumulator in the case of braking with the electrically regenerative brake.
Furthermore in the case of constant generator power, the torque which is taken up by a generator decreases significantly as the speed increases, wherein energy recovery is no longer possible below a limiting speed of approximately 15 km/h. DE 10 2004 044 599 A1 describes a method for performing closed-loop control of a brake system of a motor vehicle which has an electrically regenerative brake, in particular a generator, and a number of hydraulic friction brake driven by a brake pressure generating means via a braking medium. In the case of a decrease in the braking torque of the electrically regenerative brake which occurs, in particular, when a minimum speed is undershot, the desired overall deceleration is at least partially maintained in that an additional build of a brake pressure for the friction brakes takes place with a further auxiliary-force-assisted brake pressure generating means, for example an electric hydraulic pump.
WO2006/076999 A1 discloses a method for controlling a brake system of a motor vehicle which has an electric generator and a number of friction brakes which are assigned to axles of the motor vehicle, and the overall deceleration of which is composed of deceleration components of the electric generator and of the friction brakes. The ratio of the setpoint deceleration components of a number of brakes with respect to one another is changed by means of a control device in that a correction value for a number of the brakes is determined from the ratio of the actual deceleration to the setpoint deceleration, which correction value is applied to the setpoint deceleration of these brakes. This method compensates, for example, for ageing of the friction brakes.
If a brake process is started in the case of a vehicle speed of 100 km/h, only a small regenerative braking deceleration is available. If the ratio between the regenerative braking torque and the braking torque generated by friction brakes is constant over the entire braking process, this restricts the recovered electrical energy, i.e. the efficiency level of the energy recovery, also referred to as recuperation, is low. In the case of an electro-hydraulic regenerative brake system, the activation of the brake pedal and the pressure build up are energetically separated and therefore the braking force distribution between the generator and the friction brakes is adapted continuously, wherein the hydraulically built-up friction braking torque is therefore reduced. If a conventional brake system is used, a simple reduction of pressure at the friction brakes is, however, not readily possible.
The object of the present invention is therefore to increase the efficiency level of the recuperation when using conventional friction brakes. The efficiency level of the recuperation is given by the ratio of recovered electrical energy and the total kinetic energy of the vehicle converted during the braking process.
This object is achieved by means of the method according to this invention.